BILBAO, Spain -- Frank O. Gehry spent six years with a masterpiece in the making. He dreamed it and nurtured it, watching as it rose along a bend of a muddy river in this rust bucket of a faded industrial city.

And now, he has to let it go.

When the Guggenheim Museum Bilbao opens to the public Sunday, Gehry, a rumpled, 68-year-old prize-winning architect, will walk away from a building that some critics are hailing as one of the century's greatest.

"When I finish a project, it belongs to someone else," Gehry says. "And I have to keep my mouth shut."

Gehry's architectural creation is perhaps like nothing you've ever seen. And it is crammed with some of the great works of contemporary art culled from the vast collection of the New York-based Solomon R. Guggenheim Foundation.

The $100 million Guggenheim Museum Bilbao is perched above train tracks, next to a ramshackle storage yard, stretching underneath one of the city's busiest traffic bridges.

It's a 257,000-square-foot jumble of curves and shapes, built of Spanish limestone, glass and titanium that appears silver in the sun and golden in the rain.

The building answers a question that Gehry often posed to himself during a process of creation that he has likened to "watching grass grow."

"Should a museum be a neutral container, totally deferential to art, or what?" Gehry says. "Artists told me they were tired of neutral things. They wanted to be in an important place."

A 165-foot-high central atrium that twists to a flower-shaped roof anchors the interior. A walk through the 19 galleries on three floors is like a journey through the belly of a great sculpture.

One gallery, dubbed "The Boat," is longer than a football field and seems to swallow whole Richard Serra's 174-ton rusted-steel sculpture "Snake."

The opening show -- "The Guggenheim Museums and the Art of This Century" -- is a greatest-hits display, from the cubism of Pablo Picasso to the installation art of Jenny Holzer. It pays homage to Marc Chagall, Jackson Pollock and Andy Warhol.

And in the main plaza, outside the building, is a new work by Jeff Koons, a gigantic flower-covered sculpture titled "Puppy."

"It's a bundle of love," Koons says.

The museum sure is an unusual work for a city built on steel.

But these are new times. In America, cities often erect stadiums and lure sports teams in a bid to become "big league" and generate economic development.

In a place like Bilbao, they've followed another path to respectability and regeneration: great art and great architecture. The museum is the jewel of a $1 billion redevelopment program. Bilbao has built a subway system, a pedestrian bridge over the Nervion river, a convention center and music hall. A library and river-front business park are also planned, along with an enlarged airport.

In the 1980s, Bilbao, the northern Spanish city that faces the Bay of Biscay, was in decline. Its steel mills and shipyards were shutting. Political violence also flared in this seat of the Basque country, where a tiny minority of people support a separatist terror movement known as ETA, which translated from Spanish means Basque Homeland and Liberty. The 29-year terror campaign has led to more than 800 deaths.

The ETA has vowed to bomb the museum before Sunday's opening. This week, a policeman died after he was shot in front of the museum. Security forces also said they uncovered a weapons cache in a van near the museum.

But Bilbao is undeterred. King Juan Carlos and Queen Sofia of Spain are due in Bilbao tomorrow to celebrate the opening of the museum. Yesterday, tens of thousands of people marched in the city to protest the killing of the police officer.

"We will not take a step backward," says Josu Bergara, the chairman of the Biscay Executive Council, who vows that the museum will open as scheduled.

A step backward would be costly. It was commerce, pure and simple, that brought together the Guggenheim and Bilbao. "This is not a cultural project -- it's an economic development project," says Thomas Krens, the former college professor who directs the Guggenheim Foundation.

Krens long sought to expand his organization's reach from the Frank Lloyd Wright-designed Solomon R. Guggenheim Museum in New York City. Some have dubbed his efforts McGuggenheim, a franchising of art. He says he needs the extra exhibition space to complement the museum's headquarters, SoHo gallery in New York and collection in Venice, Italy.

After a deal to expand to Austria fell apart, Krens looked for a new partner.

Bilbao and the Basque regional government stepped forward in 1991. In an agreement that could stretch 75 years, the Guggenheim agreed to manage the museum and fill it with treasures rotated from the foundation's permanent collection. In return, the Basques gave the Guggenheim a one-time $20 million fee, provided $50 million for acquisitions, and agreed to subsidize the Bilbao museum's $12 million annual budget.